Biomass has been the primary source of energy over most of human history. During the 1800's and 1900's the proportion of the world's energy sourced from biomass dropped sharply, as the economical development of fossil fuels occurred, and markets for coal and petroleum products took over. Nevertheless, some 15% of the world's energy continues to be sourced from biomass, and in the developing world, the contribution of biomass to the energy supply is close to 38%.
Solid biomass, typically wood and wood residues, is converted to useful products, e.g., fuels or chemicals, by the application of heat. The most common example of thermal conversion is combustion, where air is added and the entire biomass feed material is burned to give hot combustion gases for the production of heat and steam. A second example is gasification, where a small portion of the biomass feedstock is combusted with air in order to convert the rest of the biomass into a combustible fuel gas. The combustible gas, known as producer gas, behaves like natural gas but typically has between 10 and 30% of the energy content of natural gas. A final example of thermal conversion is pyrolysis where the solid biomass is converted to liquid and char, along with a gaseous by-product, essentially in the absence of air.
In a generic sense, pyrolysis is the conversion of biomass to a liquid and/or char by the action of heat, normally without using direct combustion in a conversion unit. A small quantity of combustible gas is also a typical by-product. Historically, pyrolysis was a relatively slow process where the resulting liquid product was a viscous tar and a “pyrolygneous” liquor. Conventional slow pyrolysis has typically taken place at temperatures below 400° C. and at processing times ranging from several seconds to minutes. The processing times can be measured in hours for some slow pyrolysis processes used for charcoal production.
A more modern form of pyrolysis, termed fast pyrolysis, was discovered in the late 1970's when researchers noted that an extremely high yield of a light pourable liquid was possible from biomass. In fact, liquid yields approaching 80% of the weight of the input woody biomass material were possible if the pyrolysis temperatures were moderately raised and the conversion was allowed to take place over a very short time period, typically less than 5 seconds.
The homogeneous liquid product from fast pyrolysis, which has the appearance of espresso coffee, has since become known as bio-oil. Bio-oil is suitable as a fuel for clean, controlled combustion in boilers, and for use in diesel and stationary turbines. This is in stark contrast to slow pyrolysis, which produces a thick, low quality, two-phase tar-aqueous mixture in very low yields.
In practice, the fast pyrolysis of solid biomass causes the major part of its solid organic material to be instantaneously transformed into a vapor phase. This vapor phase contains both non-condensable gases (including methane, hydrogen, carbon monoxide, carbon dioxide and olefins) and condensable vapors. It is the condensable vapours that constitute the final liquid bio-oil product and the yield and value of this bio-oil product is a strong function of the method and efficiency of the downstream capture and recovery system. The condensable vapors produced during fast pyrolysis continue to react in the vapour phase, and therefore must be quickly cooled or “quenched” in the downstream process before they can deteriorate into lower value liquid and gaseous products. As fast pyrolysis equipment is scaled up in commercial operations, particular attention must be given to the strategy and means of rapid cooling, quenching and recovery of the liquid bio-oil product.